Earthquakes occur along faults in response to plate tectonic movements, but paradoxically, are not widely recognized in the geological record, severely limiting our knowledge of earthquake physics and hampering accurate assessments of seismic hazard. Light-reflective (so-called mirror like) fault surfaces are widely observed geological features, especially in carbonate-bearing rocks of the shallow crust. Here we report on the occurrence of mirrorlike fault surfaces cutting dolostone gouges in the Italian Alps. Using friction experiments, we demonstrate that the mirror-like surfaces develop only at seismic slip rates (similar to 1 m/s) and for applied normal stresses and sliding displacements consistent with those estimated on the natural faults. Under these experimental conditions, the frictional power density dissipated in the samples is comparable to that estimated for natural earthquakes (1-10 MW/m(2)). Our results indicate that mirror-like surfaces in dolostone gouges are a signature of seismic faulting, and can be used to estimate power dissipation during ancient earthquake ruptures

Previous studies show that pulverized rocks observed along large faults can be created by single high‐strain rate loadings in the laboratory, provided that the strain rate is higher than a certain pulverization threshold. Such loadings are analogous to large seismic events. In reality, pulverized rocks have been subject to numerous seismic events rather than one single event. Therefore, the effect of successive “milder” high‐strain rate loadings on the pulverization threshold is investigated by applying loading conditions below the initial pulverization threshold. Single and successive loading experiments were performed on quartz‐monzonite using a Split Hopkinson Pressure Bar apparatus. Damage‐dependent petrophysical properties and elastic moduli were monitored by applying incremental strains. Furthermore, it is shown that the pulverization threshold can be reduced by successive “milder” dynamic loadings from strain rates of ~180 s−1 to ~90 s−1. To do so, it is imperative that the rock experiences dynamic fracturing during the successive loadings prior to pulverization. Combined with loading conditions during an earthquake rupture event, the following generalized fault damage zone structure perpendicular to the fault will develop: furthest from the fault plane, there is a stationary outer boundary that bounds a zone of dynamically fractured rocks. Closer to the fault, a pulverization boundary delimits a band of pulverized rock. Consecutive seismic events will cause progressive broadening of the band of pulverized rocks, eventually creating a wider damage zone observed in mature faults.

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Thrust wedge evolution is typically characterized by out-of-sequence thrusting, which can occur in both submarine and subaerial conditions to maintain the balance between gravitational and tectonic forces. The Gran Sasso Massif, in Central Italy, is a high topography region where the kinematics and environmental conditions of deformation of some fault zones are still controversial, and this bears important implications for the Central Apennines orogenic wedge evolution. To obtain further constraints on fault activity in the Gran Sasso Massif, we studied the Monte Camicia and Vado di Ferruccio thrusts using structural, petrographical, and geochemical analyses. Such dataset allowed us to constrain the structural-diagenetic evolution of the studied faults, which has first-order implications in the characterization of their paleo-hydraulic properties. Our results indicate that in the Vado di Ferruccio out-of-sequence thrust, pressure solution-mediated mass transfer promoted low-permeability conditions in the fault core that led to a semiclosed fluid circulation, pore fluid overpressuring, and dolomite crystallization in submarine conditions. In contrast, the Monte Camicia out-of-sequence thrust was characterized by dominant cataclasis in subaerial conditions, which facilitated meteoric fluid infiltration. By considering both fault zones as belonging to the same thrust system at crustal scale, we interpret these differences as indicating the occurrence of multiple pulses of thrusting during the exhumation of this sector of the Central Apennines, up to Quaternary times, when extensional faulting eventually dissected the thrust stack. This caused extensional reactivation of the Monte Camicia Thrust and related alteration in vadose zone conditions, leading to porosity enhancement, dolomite dissolution, and calcitization.

A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L'Aquila M w 6.1 2009 and Norcia M w 6.5 2016 in Central Italy. Surprisingly, within this region, fast (≈3km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite of its relevance for seismic hazard studies, the transitions from fault creep to slow and fast seismic rupture propagation are still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures, are obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3-5 km depth (i.e., P = 100 MPa and T = 100 o C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of regular seismicity in Central Italy. The transition from slow to fast rupture is explained by the increase of the ambient temperature, which enhances the elastic loading stiffness of the fault and consequently the slip velocity during the nucleation stage, allowing flash weakening. The activation of such weakening induces the propagation of fast ruptures radiating intense high frequency seismic waves.